Composition and process for preparing cross-linked fluoro-polymer products

ABSTRACT

A CURABLE FLUOROPOLYMER COMPOSITION WHICH WILL UNDERGO CROSS-LINKING WHEN HEATED CAN BE MADE BY MIXING A COPOLYMER OF TETRAFLUOROETHYLENE AND A PERFLUOROALKYL PERFLUOROVINYL ETHER WITH A POLYFUNCTIONAL TERTIARY AMINE. THE COMPOSITION CAN BE USED TO MAKE CURED OR CROSSLINKED CHEMICAL AND HEAT-RESISTANT FLUOROPOLYMER PRODUCTS HAVING AN UNUSUAL COMBINATION OF PROPERTIES INCLUDIN INSOLUBILITY IN SOLVENT FOR THE UNCURED COPOLYMER, AND THE ABILITY TO BE MOLDED OR REMOLDED UNDER HEAT AND PRESSURE AFTER THE CROSS-LINKING OPERATION.

United States Patent O 3,686,154 COMPOSITION AND PROCESS FOR PREPARINGCROSS-LINKED FLUORO-POLYMER PRODUCTS Ausat Ali Khan, Newark, DeL,assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. NoDrawing. Filed Mar. 29, 1971, Ser. No. 129,179 Int. Cl. C08f 15/02,27/08 US. Cl. 260--87.5 A 10 Claims ABSTRACT OF THE DISCLOSURE A curablefluoropolyrner composition which will undergo cross-linking when heatedcan be made by mixing a copolymer of tetrafluoroethylene and aperfluoroal-kyl perfiuorovinyl ether with a polyfunctional tertiaryamine. The composition can be used to make cured or crosslinked chemicaland heat-resistant fluoropolyrner products having an unusual combinationof properties including insolubility in solvents for the uncuredcopolymer, and the ability to be molded or remolded under heat andpressure after the cross-linking operation.

BACKGROUND OF THE INVENTION This invention relates to fluoropolyrnercompositions which contain a copolymer of tetrafluoroethylene and aperfluoroalkyl perfluorovinyl ether, and which can be molded under heatand pressure to form products having good chemical resistance and heatresistance. The invention also concerns a process for curing suchcompositions and the resulting products.

There is a need in the art for a fluoropolyrner composition made from acopolymer of the type mentioned above which can be cured in a practicalmanner to form crosslin-ked polymeric products having (a) good chemicalresistance including insolubility at ordinary ambient temperatures insolvents which dissolve the uncured copolymer and (b) good resistance todegradation when exposed to high temperatures (e.g. 250-350 C.). Thereis also a need for such a composition which has suflicient thermoplasticproperties after the curing operation so that it can be molded,remolded, postformed, or otherwise made into shaped articles with theaid of heat and pressure.

SUMMARY OF THE INVENTION Expressed broadly, the present inventionprovides a fluoropolymer composition comprised of a mixture of (A) acopolymer of tetrafluoroethylene and perfluoroalkyl perfluorovinylether, said ether having the formula C F -O-CF=CF wherein n is a numberof 1-5, and (B) a compound selected from the group: polyfunctionaltertiary amines and precursors thereof capable of forming such amines insitu, component B being present in an amount equal to about l-10% byweight of component A.

The invention also provides a composition obtained by heating thecomposition described in the previous paragraph at a temperature ofaboutlOO-350" C. until the copolymer has undergone cross-linking.

In addition, there is provided a process for preparing a cross-linkedfluoropolyrner which comprises (1) Providing a mixture of a copolymer oftetrafluoroethylene and perfluoroalkyl perfiuorovinyl ether, said etherhaving the formula C F -O-CF=CF wherein n is a number of l-5, and acompound selected from the group: polyfunctional tertiary amines andprecursors thereof capable of forming such amines in situ, said compoundbeing present in an amount equal to about 110% by weight of thecopolymer, and

(2) Heating the resulting mixture at a temperature of ice about l00350C. until the copolymer has undergone the desired amount ofcross-linking.

DESCRIPTION OF PREFERRED EMBODIMENTS In many cases, it is preferred thatthe novel curable composition contains a component A copolymer whereinsaid ether is perfluoromethyl perfluorovinyl ether. Useful results canalso be obtained when the perfluoromethyl group of the ether is replacedwith a C -C perfluoroalkyl group. It is also preferred in manyapplications of the invention to use a component A copolymer which is anelastomer having a tetrafluoroethylene/ ether molar ratio of about 50:50to 70:30. The composition tends to become less elastomeric as the othercontent is reduced.

The component A type of copolymer is known in the art. For example,polymers of this type and methods of preparing them are described in US.Pat. 3,132,123 issued May 5, 1964 to Harris and McCane, and in CanadianPat. 894,898, issued Mar. 7, 1972 to G. A. Gallagher (priority date ofFeb. 18, 1970). The copolymer is a solid (non-liquid) under normalatmospheric conditions. The copolymer is preferably prepared by afree-radical polymerization in aqueous emulsion whereby the monomerunits are randomly connected to each other to form chains of recurringmonomer units.

The curable mixture of components A and B preferably contains about 25%of component B based on the weight of component A.

An especially preferred component B compound is triethylenediamine.Another very useful compound is N,N'- bis-(3-aminopropyl)piperazine.Other useful component B compounds are illustrated as follows: salts oftriethylenediamine (e.g. the sulfates, chlorides, and borates) which arecapable of forming the tertiary amine in situ during heating of thecomposition; tris(dodecyl methylene)diamine;3-(l,5-diazobicyclo)(3,2,1)-oct-8-yl indole; 4,4-methylene-bis-(N,N-dimethyl aniline);2,3-bis-(2-pyridyl)-5,6-dihydropyrazine; 4,4'-trimethylene dipyridine;4,4 trimethylene bis (N piperidine ethanol); N,N'- bis-(R) piperazineswherein R is a C -C alkyl group or a substituted analog thereof (e.g.containing an amino, halide, or hydroxy substituent); and Troegers base,which is also known as 2,8 dimethyl 6H, 12H-5,11 methanodibenzo [b,f][1,5] diazocine.

In preparing the cross-linked polymeric product by the process of thisinvention, it is often preferred to subject the curable mixture tosufficient pressure in a molding means during at least a portion of thestep 2 heating operation so that a cross-linked article of predeterminedshape is obtained. The molding means can be any suitable apparatus knownto be useful for the manufacture of molded articles or sheet materialsfrom polymeric materials capable of flowing under heat and pressure.

It is a preferred embodiment of the present process to heat the curablecomposition in step 2 until the copolymer has become sufficientlycross-linked so that it is substantially insoluble at ordinary ambienttemperatures (e.g. up to about C.) in a liquid organic solvent whichwill dissolve the copolymer used in step 1.

It is also preferred to heat the composition until is has undergone thedesired increase in stress/strain and flow resistance properties asmeasured at ordinary ambient temperatures. These embodiments areillustrated in the examples that follow.

The curable composition can contain one or more additives such as thoseknown to be useful in fluoropolyrner molding compositions, for example,pigments, fillers, other fluoropolymers, pore-forming agents,plasticizers such as certain fluorocarbon oils, and divalent metaloxides and hydroxides.

Fluoropolymer compositions are obtainable according to the presentinvention which are beneficially useful in many applications; forexample: (a) where a cured fiuoropolymer product is desired which hasnot only useful stress/strain and flow resistance properties at ordinarytemperatures, but also excellent chemical resistance, includinginsolubility in solvents for the uncured material, at least at ordinaryambient temperatures; (b) where a cured fluoropolymer product havingthermoplastic properties at elevated temperatures is desired so that theconfiguration of the product can be changed by known heating or hotpressing operations such as molding, remolding, postforming, and thelike; (c) where it is desired to convert an uncured fiuoropolymermaterial to a cured chemical-resistant product in a simple molding andheating operation; ((1) where an elastomer product is desired whichretains useful strength and other physical properties after extendedaging at a very high temperature followed by cooling to ordinary ambienttemperatures; and (e) where combinations of two or more suchcharacteristics are desired.

The invention makes it possible to manufacture molded fluoroelastomerarticles from the copolymer described above which have satisfactorysolvent resistance, tensile strength, heat resistance, and otherproperties for many applications where it was previously necessary touse other fiuoroelastomers (such as certain terpolymers) which are notas easy and economical to prepare as the present copolymer.

The cured compositions and the uncured compositions provided by thisinvention are useful articles of commerce which can be supplied tomanufacturers of final products and intermediate products in the generalfield of producing fluoropolymer articles.

Example 1 This example and those that follow illustrate the invention;amounts are by weight unless otherwise indicated.

A fluoropolymer composition which can be cured by cross-linking at anelevated temperature is prepared by mixing the following ingredients ona two-roll rubber mill whose rolls are at about 15.6 C.: 75 parts offiuoropolymer, parts of type SAF carbon black, 2 parts of magnesiumoxide, and 3 parts of triethylenediamine. The fluoropolymer is anelastomeric copolymer of 61 mole percent of tetrafluoroethylene and 39mole percent of perfluoromethyl perfiuorovinyl ether.

The resulting fluoroelastomer composition has the beneficial utilitydescribed in the 3 paragraphs just before Example 1.

Samples of the composition are molded, cured, and tested as describedbelow. The test results are shown in Table I.

post-curing oven. It can be seen from Test 1.2 of Table I that theelastomer product retains very useful strength and other physicalproperties after prolonged aging at 288 C., which is quite severeexposure for an elastomer.

When samples of the uncured elastomer composition as removed from thetwo-roll mill are tested for solvent resistance by leaving them immersedfor seven days at 24 C. in a solvent composed of 99%2,3-dichlorooctafiuorobutane and 1% diglyme (diethylene glycol dimethylether), the elastomer is found to be completely soluble in the solvent.In fact, the uncured elastomer dissolves within 24 hours. But whensamples of composition are molded and cured as described for Test 1.1and tested for solvent resistance in the same manner, the elastomer isfound to be insoluble in the solvent. The uncured composition also doesnot have useful stress/strain and flow resistance properties; but it isapparent from Table I that the cured material does.

Example 2 Cured elastomer samples for the Test 2.1 shown in Table I areprepared and tested in the manner described above for Test 1.1 exceptthe ingredients mixed on the rubber mill are as follows: 100 parts ofthe fiuoropolymer, 14 parts of SAP carbon black, 2 parts of magnesiumoxide, and 4 parts of triethylenediamine. As in Example 1, the elastomerof the cured samples is not soluble in the solvent.

Some of the molded samples as prepared for Test 2.1 are subjected to aremolding operation by cutting the samples into pieces about A inchsquare, placing a quantity of the pieces between two sheets ofpolytetratluoroethylene in a hydraulic press, and molding the piecesinto a /s inch thick sheet by using a molding time/temperature/ pressureof 30 minutes/204 C./ 10,000 pounds total pressure. The remolded samplesare tested in the manner described for Test 1.1, and the results arereported in Table I as Test 2.2. The composition has surprisingly goodremolding properties considering that it has been crosslinked andrendered insoluble; and its Table I physical properties are unexpectedlygood for a remolded fiuoroelastomer. The product has a set at breakvalue of 5% when the remolded samples are tested at 121 C.

Example 3 Another useful composition is prepared in the manner describedin Example 2 except (a) the triethylenediamine is replaced with 5 partsof tris(dodecyl methylene) diamine. Samples of the composition aremolded for 30 minutes at 180 C, and post-cured in an oven filled withnitrogen while the temperature rises to 204 C. during a pe- The samplesfor Test 1.1 are compression molded for 30 minutes at 180 C. andpost-cured in an oven in which a blower circulates the air. Duringpost-curing, the molded samples are heated as follows: 24 hours at 149C., 24 hours at 177 C., 24 hours at 204 C., 6 hours while thetemperature rises to 300 C., then for 24 hours at 300 C. A simplercuring cycle is shown below in Examples 3-5.

The modulus, tensile, elongation, and set values in Table I are obtainedat room temperature by AST M Method D412-66.

The samples for Test 1.2 are prepared by repeating the Test 1.1 moldingand post-curing procedure, followed by Example 4 A usefulfiuoroelastomer composition is prepared in the manner described inExample 2 except the triethylenediamine is replaced with 3 parts ofN,N'-bis-( 3 aminopropyl) piperazine. Samples of the composition aremolded for 30 heat aging the samples for seven days at 288 C. in theminutes at 180 C. and post-cured in an oven while the temperature risesto 232 C. during a period of 4 hours and then remains at 232 C. for 24hours.

The resulting cured samples are tested as described for Test 1.1, andthe results are reported as 4.1 in Table I. Even after the cured samplesare subjected to a severe heat exposure cycle of 65 hours at 316 C. in anitrogen-filled oven, followed by cooling to 24 C., they still havephysical properties which are useful in many applications as shown byTest 4.2 in Table I.

Example 5 Example 4 is repeated except the 3 parts of N,N'-bis-(3-aminopropyDpiperazine are replaced with 4 parts of the same compound,and the post-curing temperature is 204 C. instead of 232 C. The testresults are reported as Test 5.1 in Table I. The elastomer of the curedsamples is insoluble in the solvent used in Example 3 after immersiontherein for seven days at 24 C.

I claim:

1. A fluoropolymer composition comprised of a mixture of (A) a copolymerof tetrafluoroethylene and a perfluoroalkyl perfiuorovinyl ether, saidether having the formula C F O-CF=CF wherein n is a number of 1-5, and

(B) a compound selected from the group: polyfunctional tertiary aminesand precursors thereof capable of forming such amines in situ, componentB being present in an amount equal to about l% by weight of component A.

2. A composition obtained by heating the composition of claim 1 at atemperature of about 100-350" C. until the copolymer has undergonecross-linking.

3. A composition according to claim 1 which contains about 2-5 ofcomponent B based on the weight of component A.

4. A composition according to claim 3 wherein said ether of component Ais perfluoromethyl perfluorovinyl ether.

5. A composition according to claim 3 wherein component A is anelastomer having a tetrafluoroethylene/ether molar ratio of about :50 to:30.

6. A composition according to claim 3 wherein component B istriethylenediamine.

7. A composition according to claim 3 wherein component B is N,N'-bis-(3 aminopropyDpiperazine.

8. A process for preparing a cross-linked fluoropolymer which comprises(1) providing a mixture of a copolymer of tetrafluoroethylene and aperfluoroalkyl prefluorovinyl ether, said ether having the formula C F-O-CF=CF wherein n is a number of 1-5, and a compound selected from thegroup: polyfunctional tertiary amines and precursors thereof capable offorming such amines in situ, said compound being present in an amountequal to about 1-10% by weight of the copolymer, and

(2) heating the resulting mixture at a temperature of about IOU-350 C.until the copolymer has undergone the desired amount of cross-linking.

9. A process according to claim 8 wherein the mixture is subjected tosufiicient pressure in a molding means during at least a portion of step(2) so that a cross-linked article of predetermined shape is obtained.

10. A process according to claim 8 wherein the mixture is heated in step(2) until the copolymer has become sufficiently cross-linked so that itis substantially insoluble at ordinary ambient temperature in a liquidorganic solvent which will dissolve the copolymer used in step (1).

References Cited UNITED STATES PATENTS 2,793,202 5/1957 West 260-875 A2,979,490 4/ 1961 West 26087.5 A 3,132,123 5/ 1964 Harris et al 26087.5A

JOSEPH L. SCHOFER, Primary Examiner J. A. DONAHUE, JR., AssistantExaminer

